fix(tui): restore prompt rendering on startup (#498)

* fix(tui): restore prompt rendering on startup

* test(tui): document render-time command split

* fix(tui): reduce ghostty prompt repaint scope

src/ink/log-update.test.ts

@@ -0,0 +1,125 @@
+import { expect, test } from 'bun:test'
+
+import type { Frame } from './frame.ts'
+import { LogUpdate } from './log-update.ts'
+import {
+  CellWidth,
+  CharPool,
+  createScreen,
+  HyperlinkPool,
+  setCellAt,
+  StylePool,
+} from './screen.ts'
+
+function collectStdout(diff: ReturnType<LogUpdate['render']>): string {
+  return diff
+    .filter((patch): patch is Extract<(typeof diff)[number], { type: 'stdout' }> => patch.type === 'stdout')
+    .map(patch => patch.content)
+    .join('')
+}
+
+function createHarness() {
+  const stylePool = new StylePool()
+  const charPool = new CharPool()
+  const hyperlinkPool = new HyperlinkPool()
+
+  return {
+    stylePool,
+    charPool,
+    hyperlinkPool,
+    log: new LogUpdate({ isTTY: true, stylePool }),
+  }
+}
+
+function frameFromLines(
+  stylePool: StylePool,
+  charPool: CharPool,
+  hyperlinkPool: HyperlinkPool,
+  lines: string[],
+  cursor = { x: 0, y: lines.length, visible: true },
+): Frame {
+  const width = lines.reduce((max, line) => Math.max(max, line.length), 0)
+  const screen = createScreen(width, lines.length, stylePool, charPool, hyperlinkPool)
+
+  for (const [y, line] of lines.entries()) {
+    for (const [x, char] of [...line].entries()) {
+      setCellAt(screen, x, y, {
+        char,
+        styleId: stylePool.none,
+        width: CellWidth.Narrow,
+      })
+    }
+  }
+
+  return {
+    screen,
+    viewport: {
+      width: Math.max(width, 1),
+      height: 10,
+    },
+    cursor,
+  }
+}
+
+test('ghostty main-screen rewrite paints prompt content without full terminal reset when width is stable', () => {
+  const { stylePool, charPool, hyperlinkPool, log } = createHarness()
+  const prev = frameFromLines(stylePool, charPool, hyperlinkPool, ['      '])
+  const next = frameFromLines(stylePool, charPool, hyperlinkPool, ['prompt'])
+
+  const diff = log.render(prev, next, false, true, true)
+  const stdout = collectStdout(diff)
+
+  expect(diff.some(patch => patch.type === 'clearTerminal')).toBe(false)
+  expect(diff.some(patch => patch.type === 'clear' && patch.count === 1)).toBe(
+    true,
+  )
+  expect(stdout).toContain('prompt')
+})
+
+test('ghostty main-screen rewrite clears only the changed prompt tail before repainting', () => {
+  const { stylePool, charPool, hyperlinkPool, log } = createHarness()
+  const prev = frameFromLines(
+    stylePool,
+    charPool,
+    hyperlinkPool,
+    ['status', '> abc'],
+  )
+  const next = frameFromLines(
+    stylePool,
+    charPool,
+    hyperlinkPool,
+    ['status', '> abcd'],
+  )
+
+  const diff = log.render(prev, next, false, true, true)
+  const stdout = collectStdout(diff)
+
+  expect(diff.some(patch => patch.type === 'clearTerminal')).toBe(false)
+  expect(diff.some(patch => patch.type === 'clear' && patch.count === 1)).toBe(
+    true,
+  )
+  expect(stdout).toContain('abcd')
+})
+
+test('ghostty main-screen rewrite falls back to incremental diff for larger changes', () => {
+  const { stylePool, charPool, hyperlinkPool, log } = createHarness()
+  const prev = frameFromLines(
+    stylePool,
+    charPool,
+    hyperlinkPool,
+    ['row 0', 'row 1', 'row 2', 'row 3', 'row 4', '> abc'],
+  )
+  const next = frameFromLines(
+    stylePool,
+    charPool,
+    hyperlinkPool,
+    ['row 0 updated', 'row 1', 'row 2', 'row 3', 'row 4', '> abcd'],
+  )
+
+  const diff = log.render(prev, next, false, true, true)
+  const stdout = collectStdout(diff)
+
+  expect(diff.some(patch => patch.type === 'clear')).toBe(false)
+  expect(stdout).toContain('updated')
+  expect(stdout).toContain('abcd')
+})